The present invention relates generally to the field of data communications, and more specifically, to the field of synchronous, fixed boundary, Variable data rate communication systems, such as code division multiple access (CDMA) North American digital cellular telephone systems.
Synchronous communication systems which utilize fixed frame boundary data frames including data at variable rates are known in the art. One example is the CDMA North American digital cellular system, a well-known class of modulation using specialized codes to provide multiple communication channels in a designated segment of the electromagnetic spectrum. The Telecommunications Industry Association (TIA) has standardized a CDMA implementation in the "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System TIA/EIA/IS-95 Interim Standard" (IS-95) and the "Speech Service Option Standard for Wideband Spread Spectrum Digital Cellular System TIA/EIA/IS-96 Interim Standard" (IS-96). Sections 6-6.2.4 and 7-7.2.4 of IS-95 and the entire IS-96 are hereby incorporated by reference.
In the conventional CDMA digital cellular system, as disclosed in the current IS-95 and IS-96, variable data rates are utilized by reducing the data transmission rate during times of reduced speech activity. This data rate reduction results both in a reduction of interference with other users (thereby increasing capacity in the system) as well as in a reduction in average transmit power of the CDMA mobile station (thereby increasing battery life). On the transmitter end (transmitting base station or transmitting mobile station), a vocoder (voice or speech encoder/decoder) compares voice energy levels to adaptive thresholds based on background noise levels to determine an appropriate data rate for each frame of speech data, thereby suppressing background noise and providing good voice transmission in noisy environments. Using a code excited linear prediction (CELP) method, the vocoder receives pulse code modulated speech samples and reduces the number of bits required to represent speech by exploiting the intrinsic properties of speech signals to remove redundancy. Subsequently, the speech encoded data is convolutionally encoded for forward error correction before being interleaved and modulated for transmission.
Since the data rate may change at each frame boundary, the CDMA receiver must first determine the data rate of each frame of data. The process by which this is accomplished in the conventional CDMA digital cellular system is a source of wasted time and processing energy. According to the current CDMA digital cellular system, each data frame must be separately processed at each of the various possible data rates (including convolutional decoding) before a decision is made regarding which data rate was utilized on the transmitter end. Since this method is clearly inefficient, there is a need in the industry for a new method for determining the data rate of each frame of data in the CDMA digital cellular system, as well as other systems using fixed boundary frames with variable data rates.
One possible method of addressing this problem is the addition of a conventional header before each frame of data. Such a header could include the data rate of the corresponding frame to which it is attached. Unfortunately, such a header would also need error protection to reduce the likelihood of transmission errors. In view of the relatively small size of each frame of data, the additional bits required for an error protected header would certainly add substantial overhead and undesirable complexity to the system.
There is, therefore, a need in the industry for a system which addresses these and other related, and unrelated, problems.